It is known, for example, from European Patent No. 0 017 793, how to improve the printability of paper by incorporating hydratable, film-forming colloidal clays, e.g. bentonite, attapulgite or sepiolite, into the paper pulp. Also, macromolecules of polyglycol, having a molecular weight of 5,000 to 100,000 can be attached to these colloidal clays. The improvement in the calendering and printing properties brought about by these measures consists in an improved "ink hold," i.e. the printing ink does not penetrate so quickly in a short time (between its application on the paper and its drying); instead, the same ink contours as they are applied on the paper are present also on the finished printed and dried paper. If the "ink hold" is poor, the ink penetrates and diffuses into the paper in a wicking action in which it spreads in the paper, resulting in an irregular and unsharp and usually dull graphic picture. The main reason for the improved ink hold is seen in that the hydratable, film-forming, colloidal clays contain a considerable percentage of bound water. At the drying temperatures normally used in a paper machine, this water cannot escape, and as it is not miscible with the solvent of the intaglio ink, it causes a repulsion of the ink, as it were.
When a mixture of colloidal clays and polyglycols is used, it is assumed that the polyglycols, like the water, become embedded between the colloidal clays, hence do not form reaction products, and that because of their wax-like constitution, they improve calenderability of the paper after the drying. A reaction with the organic solvent, in which the printing ink is dissolved or dispersed, does not occur.
The purpose of the present invention is to improve the holdout of organic solvent systems, such as printing inks, lacquers and coating substances, by other means. The problem of holdout is especially pronounced in intaglio gravure printing methods, as intaglio inks, compared with other printing inks (for letterpress or offset printing), must have a much lower viscosity. The invention, therefore, is applicable primarily in the field of intaglio gravure printing, and therefore the following statements relate to this field. Also, the flat structures of fibers which are to be printed according to the invention involve primarily those of paper, although nonwoven materials or textiles (e.g. silk, cotton and linen fabrics) can be printed using the present invention.
Intaglio gravure printing is one of the most widely spread printing methods in mass-produced printed matter of any kind. Two paper grades are used essentially, namely:
1. the highly filled, supercalendered, usually wood-containing intaglio printing paper in weights between 40 to about 80 g/m.sup.2 and PA1 2. the coated, wood-containing or wood-free, highly-calendered intaglio printing paper in weights between 45 and about 135 g/m.sup.2. PA1 (a) a water-insoluble hydrated cation-exchangeable film-forming smectitic layered silicate having an ion exchange capacity of at least 50 milliequivalents/100 g and PA1 (b) an organic radical, derived from an onium compound attached thereto by ion bond, is introduced into the fibrous material or onto the surface of the fibrous structure so that the organophilic complex forms a barrier layer by reaction with the organic solvent.
For economic and mailing reasons, the tendency has existed for years to reduce the basic weights of such papers. This desire finds limits in particular in coated intaglio printing paper, but also in uncoated (natural) such paper. To have a good standing of the intaglio ink on the paper surface, the coating must, for the coated grades, have a minimum coating weight of about 6.5 to 7 g/m.sup.2 per side; for intaglio printing paper coated on both sides, there results from this at a total weight of approximately 50 g/m.sup.2, a raw paper to be coated of about 36 g/m.sup.2. In light of today, this is a lower limit, as it is only the fiber bonds of the raw paper that contribute to the physical strength values of the printing paper.
On the other hand, the uncoated, natural intaglio printing papers are not equivalent either in whiteness or in the gloss of the producible printed matter to the coated intaglio printing papers. The consumption of intaglio ink is about two and a half to three times that of the coated papers, because the porosity and hence the absorbancy of the natural intaglio gravure papers is substantially greater. Consequently, the strike through of the print on the back (the so-called print opacity) is a special problem with these papers if the weight is further reduced.
Through the use of hydratable film-forming colloidal clays described in the above mentioned European Patent No. 0,017,793, it has indeed been possible to a certain degree to close the surface of the uncoated intaglio gravure papers somewhat and to improve the printability. However, these thus treated gravure papers do not even approximately compare with the coated intaglio gravure papers in ink absorption. However, use of the hydrated film-forming clays described in EUP 0,017,793 in coating formulations or as a surface coat is impossible for rheological reasons.